Structure of benzene

Benzene's molecular formula, C6H6, carries a high degree of unsaturation on paper, and both open-chain and cyclic structures with double and/or triple bonds could in principle be drawn for it. But benzene's actual chemical behaviour rules out any such structure: as Table 15.7 shows directly, an alkene decolourises dilute alkaline KMnO4, decolourises Br2 in CCl4, and adds water in acidic medium, while benzene does none of these three things at all. This total absence of typical alkene-style reactivity, despite benzene's formula suggesting three degrees of unsaturation, is the first and clearest piece of evidence that benzene cannot have an ordinary open-chain polyene structure. Further evidence points specifically to a symmetric six-membered ring: benzene reacting with equimolar bromine (FeBr3 catalyst) gives only ONE monosubstituted product (C6H5Br), with no isomers at all, which is possible only if all six hydrogens (and, by extension, all six carbons) in benzene are completely equivalent to one another -- exactly what a symmetric six-carbon ring, each carbon bearing one hydrogen, would produce. Catalytic hydrogenation of benzene over nickel gives cyclohexane (adding 3 H2), independently confirming both the six-membered ring and the presence of three double-bond-equivalents of unsaturation. Kekule (1865) proposed the structure that follows most directly from this evidence: a planar, six-membered ring of carbon atoms with alternating single and double bonds, one hydrogen on each carbon. This simple alternating structure predicts that there should be two distinguishable isomers of 1,2-dibromobenzene (bromines on a formally double-bonded carbon pair, versus on a formally single-bonded pair), but in reality only ONE ortho-dibromobenzene is ever isolated. Kekule patched this discrepancy by proposing that the double bonds 'oscillate' rapidly between the two possible positions, but even this refinement still could not explain benzene's unusually low reactivity or its strong preference for substitution over addition -- an explanation that had to wait for resonance theory. In the resonance picture, benzene is understood as a hybrid of the two Kekule structures (which differ only in which alternating set of carbons carries the double bonds); the true molecule is more stable than either individual resonance structure, and this extra resonance stabilisation is strong enough that the ring's pi electrons resist the localisation needed for an ordinary addition reaction, directly explaining the lack of reactivity seen in Table 15.7. The modern orbital picture makes this concrete: all six ring carbons are sp2-hybridised, using two of their three sp2 orbitals to form the six ring C-C sigma bonds and the third to form a C-H sigma bond, which leaves one unhybridised p-orbital standing perpendicular to the ring plane on every carbon. These six parallel p-orbitals overlap laterally all the way around the ring, and because this overlap can be drawn equally validly as either of two alternative pairings (C1-C2/C3-C4/C5-C6, or C2-C3/C4-C5/C6-C1), molecular orbital theory describes the resulting six pi electrons as genuinely delocalised over the whole ring, occupying a lowest-energy pi molecular orbital that forms two continuous rings of electron density, one above and one below the plane of the six carbons. This deloca …